Minimally invasive medical procedures involve the delivery and manipulation of drugs, tools and prosthetic devices inside the body while minimizing or avoiding damage to surrounding tissue structures. Entry to the body may be percutaneous or through an orifice. The surgical target may lie in the interior of a solid tissue structure or inside a body cavity. In many cases, the need arises during insertion to steer along three-dimensional curves through tissue to avoid bony or sensitive structures (percutaneous procedures), or to follow the interior contours of a body orifice (e.g., the nasal passages) or body cavity (e.g., the heart).
Some procedures, such as drug delivery, do not require articulation of the instrument's distal tip while for others it is necessary to control the position and orientation of the distal tip while holding relatively immobile the proximal inserted length. An example of the latter case arises in the navigation and articulation of interventional tools in minimally invasive intracardiac reconstructive surgery and arrhythmia management. Minimally-invasive beating-heart repair of congenital cardiac defects in children can require the use of instruments with diameters of 3-5 mm while the repair of fetal defects in utero can require the use of instruments having diameters of one millimeter or less.
The instruments used in minimally invasive procedures typically fall into one of three general categories. The first category includes straight flexible needles which are used for percutaneous procedures in solid tissue. Steering of the straight flexible needles along a curved insertion path is achieved by applying lateral forces at the needle base or tip as the practitioner inserts and advances the needle into the tissue. The lateral forces cause the partially inserted needle to flex in a desired steering direction. Base steering is accomplished by lateral displacement or rotation of the uninserted portion of the needle while tip steering is accomplished using either a beveled or curved tip, both of which produce a lateral steering force in the direction of the bevel or curve. The steering direction is adjusted by rotating the needle about its axis. Since the needle is initially straight, both base and tip steering methods rely on tissue reaction forces to flex the needle along a curved insertion path. Consequently, these instruments possess no ability to produce lateral tip motion without further penetration into solid tissue.
The second category of instruments includes a straight, fairly rigid shaft with an articulated tip-mounted tool (e.g., forceps). Both hand-held and robotic instruments of this type are in common use for minimally invasive access of body cavities (e.g., chest or abdomen). The shaft must follow a straight-line path from the entry point to the body to the surgical site. Lateral motion of the tip depends on pivoting the straight shaft about a fulcrum typically located at the insertion point into the body. This pivoting motion produces tissue deformation proportional to the thickness of tissue from the entry point to the body cavity.
The third category of instruments includes elongated, steerable devices, such as multi-stage micro-robot devices, which are typically used for entry through a body orifice, and steerable catheters, which are most often used for percutaneous access of the circulatory system. Multi-stage micro-robot devices are typically mounted at the distal end of a rigid shaft and include a flexible backbone having a series of regularly spaced platforms that are interconnected by hinges and attached to a base by wires. User actuation of the wires causes the backbone to bend. These devices are typically sufficiently rigid to support their own weight as well as to apply appreciable lateral forces to the surrounding tissue. In contrast, steerable catheters include an elongate member of sufficient flexibility so as to conform to the curvature of the vessel through which it is advanced. Steering is achieved using one or more wires attached to the distal end of the elongate member disposed along the catheter's length. User actuation (e.g., pulling) of the wires causes the distal portion of the elongate member to flex in one or more directions. An alternate approach to catheter steering for navigating branches in the circulatory system involves the use of a guide wire. After inserting the flexible catheter up to the branch point, the guide wire, with a curvature preset at its distal end, is inserted through the catheter until its curved portion is extended beyond the distal end of the catheter and into the desired branch vessel. At this point, pushing on the catheter from the base causes its distal end to follow the wire into the branch vessel.